esp32s2: audiobusio: move i2s_common inside

Originally, I believed the implementation might be shared with AudioOut,
as on the ESP32 (non-S2) the I2S peripheral was also used to drive the DAC.
However, this is not the case on ESP32-S2 and appears it will not be
the case with the ESP32-S3 or -C3, to the extent that there's skeletal
support for either of them in esp-idf master branch.

However, it could still be shared by I2SIn or PDMIn (the latter being
hypothetically implemented as I2SIn + digital postprocessing like we did
in the atmel-sam port, to my understanding), so I moved it to
the common-hal folder.

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